The high speed conversion of materials in continuous web form is well known. Such "converting" processes include die cutting, slitting, sheeting and placement of components on a moving web. Traditionally, each converting step is carried out at a different location or converting station located along a linear piece of production equipment. As used herein the terms "continuous web" and "web having an indefinite length" refer to webs that have a length sufficiently great relative to converting machine and process that for a reasonable period of time the web does not have an end passing through the converting machine or process.
One problem with converting equipment is controlling registration along the web between stations which are performing converting steps on the web. Registration has been achieved by using individual drives at each converting station which are controlled electronically to maintain both registration with respect to the converting steps as well as proper web tension between stations. Such systems are, however, typically expensive and may require elaborate programming to maintain proper web tension and registration between stations.
Earlier designs of converting equipment relied on mechanical registration between stations by providing a central driveshaft which was used to drive individual stations located in a linear arrangement. The length of such systems resulted in an accumulation of tolerances along the driveshaft. As a result, it was difficult to maintain those systems in registration because of the tendency of the machine to drift out of registration due to the variations induced by those tolerances. Furthermore, the mechanical registration drive control systems also suffered from a lack of independent registration control at each of the converting stations. In addition, where tension control was important, such systems required relatively elaborate and expensive tension controls including nip rolls and tension rolls.
The problems of controlling both registration and tension with any converting system are substantially increased when converting elastic and/or delicate web materials which are much more sensitive to tension variations than more stable web materials. Lack of tension control and corresponding registration problems make such materials difficult to convert and result in high waste and low productivity.
Another disadvantage of linear web converting lines, whether relying on electrical or mechanical registration control, is that they require relatively large amounts of floor space for converting processes which increases the cost of goods produced using the converting lines.
U.S. Pat. No. 4,854,983 to Bryniarski et al. discloses a method of heat sealing a web by directing it over a central roll around which a plurality of nip rolls are placed. The central roll is preferably heated at certain areas to affect heat sealing of the plastic web material in the transverse web direction. Although this design does disclose a series of nip rolls circumferentially spaced around a central roll, there is no discussion regarding registration problems because the nip rolls do not perform converting operations which require registration. Instead, heat sealing is performed by heated sections on the central roll, around which spacing is constant.
U.S. Pat. No. 5,017,184 to Takahori et al. discloses a single converting station design in which rotary registration is accomplished between a tool roll and an anvil roll through the movement of helical gears to effect a relative rotation between the tool roll and anvil roll. This system is not, however, deployed on the circumference of a central anvil roll along with other converting steps requiring registration.